I travelled on the Eurostar from Paris to London today. I note the wonderful Man in Seat 61 says to check in 30 minutes before. This would be ok for a EU national but, on my experience today, not long enough for the rest of us. I was booked on the 11.10 and arrived at check- in at 9.30. I was not through to the waiting area until after 10.15. I am not sure if a Tuesday morning in August is exceptional. There were people behind me booked on the 10.10 .The officials at the UK customs desk would not assist but people in the queue were more helpful. One girl (North American accent) was in tears behind me, told me she had only just bought her ticket (hate to think the price). Fortunately passengers allowed her and 2 other ladies to jump to the head of the queue. I saw her being asked lots of questions by the UK customs officer (he asked me, a 74 year old, a lot as well). The last I saw she was headed for the security machine at 10.05 and only had a back pack. The other 2 ladies had a less investigative officer and were probably just ahead of her. I hope she made it. I noted EU citizens sailing through in the other area.
Also I booked within 24 hours of the date opening and chose a forward facing window seat, no table. Like many others my boarding pass had seat changed. I had an aisle seat facing back with a table. However, while a young man had the window and either looked at his phone or slept all the way, fortunately we had no-one in the opposite seats so I swapped, faced forward and could look diagonally out his window. If I had had to travel trying to find leg room with a stranger opposite I would have been furious. I am returning on Sep 10 so will see what happens.
My advice is, if you are not an EU citizen arrive an hour before.